PROJECT SUMMARY Abdominal aortic aneurysm (AAA) is a vascular disease affecting millions of Americans that accounts for 15,000 deaths per year in the United States. Currently, open or endovascular surgery is the only therapeutic option for AAA, as no drug has been approved for treatment of this devastating disease, highlighting an urgent need for new mechanistic understandings of AAA. We recently reported that administration of mineralocorticoid receptor (MR) agonists, Aldo or deoxycorticosterone acetate (DOCA) plus salt, but not Aldo, DOCA or salt alone, to 10-month-old C57BL/6 male mice potently induces AAA, and our preliminary data demonstrated that administration of angiotensin II (Ang II) plus salt, but not Ang II alone, to 10-month-old C57BL/6 male mice also potently induces AAA. While the mechanism remains elusive, our preliminary data show that Aldo- or Ang II- salt-induced AAA occurs only or mostly in male but not in female or orchidectomized (orx) male mice. To elucidate the mechanism that underlies sexual dimorphism of AAA, we identified several genes that were selectively upregulated by Aldo-salt in aorta in male but not in female or orx male mice. Among these genes, circadian clock BMAL1 and inflammatory cytokine interleukin 6 (IL-6) are particularly promising as BMAL1 has not been previously implicated in AAA despite wide recognition that aortic dissection and aneurysmal rupture are characterized by circadian variation, and as IL-6 has been associated with human AAA by recent large genome-wide association studies (GWAS). Our preliminary data show that selective deletion of BMAL1 from smooth muscle (SM) completely abolished DOCA-salt-induced AAA and IL-6 upregulation. In addition, we found that treatment of mice with a novel small molecule inhibitor targeting the IL-6 signaling significantly diminished Aldo-salt-induced AAA. Based on these provocative preliminary data and the literature that testosterone (T) promotes AAA, whereas estrogen (E2) protects against AAA, we hypothesize that T and E2 critically regulate sexual dimorphism in Aldo- or Ang II-salt-induced AAA via SM-BMAL1 and IL-6 signaling, and further, that targeting IL-6 signaling represents a novel therapeutic strategy against AAA. Two specific aims test this central hypothesis: Aim1. To determine whether sex hormone is crucial for Aldo- or Ang II-salt to activate BMAL1 in aorta and thereby result in sexual dimorphism in Aldo- or Ang II-salt- induced AAA. Aim2. To define the mechanism by which IL-6 is transcriptionally regulated by SM-BMAL1 and to target IL-6 signaling as a novel therapeutic strategy against Aldo- or Ang II-salt-induced AAA. To achieve these aims, we will administer Aldo- or AngII-salt and/or T or E2 to 10-month-old normal or castrated C57BL/6 male or female mice, SM-BMAL1-KO, IL-6R-KO, and wild-type control mice to induce SM-BMAL1 and IL-6 signaling and AAA. The proposed studies will shed new mechanistic insight into a novel role of BMAL1 and IL-6 signaling in sexual dimorphism of AAA. Moreover, the results will provide preclinical evidence that targeting IL- 6 signaling represents a novel therapeutic strategy against AAA.